Contact sensor and sheath exit sensor

ABSTRACT

A system and method is provided that allows for determining the local impedance of one or more electrodes of an electrode catheter. Such local impedance may be utilized to identify the relative position of an electrode catheter to a sheath of a guiding introducer. In another arrangement, local impedance of a catheter electrode can be utilized to calibrate a catheter electrode to provide improved contact sensing.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 11/618,484 entitled “CONTACT SENSOR AND SHEATH EXIT SENSON”having a filing date of Dec. 29, 2006 and which will issue as U.S. Pat.No. 8,265,745 on Sep. 11, 2012, the entire contents of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

a. Field of the Invention

The instant invention is directed toward an electrode catheter and amethod for using the electrode catheter for tissue mapping, guidanceand/or tissue ablation. In particular, the electrode catheter of thepresent invention may assess electrode location relative to an insertionsheath and/or assess electrode-tissue contact.

b. Background Art

Catheters have been in use for medical procedures for many years.Catheters can be used for medical procedures to examine, diagnose, andtreat while positioned at a specific location within the body that isotherwise inaccessible without more invasive procedures. During theseprocedures a catheter is typically inserted into a vessel near thesurface of the body and is guided to a specific location within the bodyfor examination, diagnosis, and treatment. For example, catheters can beused to convey an electrical stimulus to a selected location within thehuman body, e.g., for tissue ablation. In addition, catheters withsensing electrodes can be used to monitor various forms of electricalactivity in the human body, e.g., for electrical mapping.

Catheters are used increasingly for medical procedures involving thehuman heart. Typically, the catheter is inserted in an artery or vein inthe leg, neck, or arm of the patient and threaded, sometimes with theaid of a guide wire or introducer, through the vessels until a distaltip of the catheter reaches the desired location for the medicalprocedure in the heart. In the normal heart, contraction and relaxationof the heart muscle (myocardium) takes place in an organized fashion aselectro-chemical signals pass sequentially through the myocardium.

Sometimes abnormal rhythms occur in the heart, which are referred togenerally as arrhythmia. The cause of such arrhythmia is generallybelieved to be the existence of an anomalous conduction pathway orpathways that bypass the normal conduction system. These pathways areusually located in the fibrous tissue that connects the atrium and theventricle.

An increasingly common medical procedure for the treatment of certaintypes of cardiac arrhythmia is catheter ablation. During conventionalcatheter ablation procedures, an energy source is placed in contact withcardiac tissue (e.g., associated with a anomalous conduction pathway) toheat the tissue and create a permanent scar or lesion that iselectrically inactive or noncontractile. The lesion partially orcompletely blocks the stray electrical signals to lessen or eliminatearrhythmia.

Ablation of a specific location within the heart requires the preciseplacement of the ablation catheter within the heart. Precise positioningof the ablation catheter is especially difficult because of thephysiology of the heart, particularly because the heart continues tobeat throughout the ablation procedures. Commonly, the choice ofplacement of the catheter is determined by a combination ofelectrophysiological guidance and computer generated maps/models thatmay be generated during a mapping procedure. Accordingly, it isdesirable that any map or model of the heart be as accurate aspracticable.

Several difficulties may be encountered, however, when attempting toform lesions at specific locations using some existing ablationelectrodes. One such difficulty encountered with existing ablationelectrodes is how to ensure adequate tissue contact and/or electricalcoupling. Electrode-tissue contact is not readily determined usingconventional techniques such as fluoroscopy. Instead, the physiciandetermines electrode-tissue contact based on his/her experience usingthe electrode catheter. Such experience only comes with time, and may bequickly lost if the physician does not use the electrode catheter on aregular basis. In addition, attempts to directly measure contact betweenan electrode and target tissue are often subject to local variations inthe surrounding media. Such variation can distort such measurement,which can lead to false electrode-tissue contact indications.

BRIEF SUMMARY OF THE INVENTION

Generally, it has been recognized that the environment surrounding acatheter electrode alters the electrical properties of the electrode inresponse to an applied electrical signal. For instance, the impedancemay vary, or a value related to impedance or impedance components mayotherwise vary, between an electrode that is within a confined/smallvolume structure (e.g., a sheath or vessel) and the same electrode in astructure having a larger volume. That is, the surrounding fluid ofsmall volume structure may have different electrical properties (e.g.,based on flow rates etc.) than the electrical properties surroundingfluid of a larger volume. These different electrical properties alterthe local electrical response of the electrode fluid interface.Accordingly, it has been recognized that changes of such local responsesmay be utilized to provide useful information that can be utilized to,for example, provide improved sensing during mapping and/or provideimproved tissue contact sensing.

In one arrangement, local responses of a catheter may be utilized toidentify the relative position of an electrode catheter to a sheath of aguiding introducer. In another arrangement, local responses of acatheter electrode can be utilized to calibrate a catheter electrode toprovide improved contact sensing.

According to one aspect, a system and method is provided for sensingwhen an electrode of a catheter passes between a constricted area and aless constricted area. For example when the catheter passes into and/orout of a sheath of an introducer or when the catheter passes into or outof a blood vessel. Initially, an introducer may be guided to an internaltissue location of interest such as the heart. The introducer mayprovide an internal channel or lumen to the tissue area of interest.Accordingly, a catheter may be disposed through the internal channel toaccess the tissue area. In conjunction with moving the catheter relativeto the introducer, an electrical signal may be provided to one or moreelectrodes associated with the catheter. Accordingly, by monitoring aresponse of these one or more electrodes, a change in the response maybe identified that is indicative of the electrode at least partiallypassing between a constricted area and a less constricted area.Accordingly, upon identifying a change in the response, an output may begenerated that is indicative of this change of response.

Identifying a change in the response may include measuring an impedanceof the electrode in response to the electrical signal. In such anarrangement, the impedance may be measured at a series of times toidentify changes therein. For instance, an initial impedance may bemeasured for the electrode when the electrode is at a first position,which may be known to be within the internal channel of the introduceror disposed in a fluid or blood pool away from a wall of internal tissuearea. Accordingly, as the location of the electrode changes, asubsequent impedance measurement may be obtained. These impedancemeasurements may be compared to identify a change between the initialimpedance and the subsequent impedance. Likewise, if the change betweenthe initial and subsequent impedances is greater than a predeterminedthreshold, which may be based on stored information, an output may beprovided to a user indicating the change. Further, the change inimpedance may be utilized to identify local variances of surroundingmedia (e.g., blood) such that the electrode may be calibrated forsubsequent tissue contact.

In a further arrangement of the present aspect, the catheter may includeat least first and second electrodes. For instance, the catheter mayinclude a tip electrode and a ring electrode or a plurality of ringelectrodes. In such an arrangement, impedances of the tip electrode andone or more of the ring electrodes may be monitored. If the surroundingenvironment of one of the electrodes changes, for example, caused bymoving the catheter, a change in the response of one of the electrodesmay occur without a corresponding change in the other electrode.Accordingly, such a change in the relative responses may be utilized toidentify the passage of one of the electrodes into or out of theintroducer or into or out of an internal structure such as a vein orartery.

According to another aspect, a system and method is provided for use incalibrating an electrode catheter as well as assessing contact betweenan electrode and tissue. The system/method includes guiding a catheterto an internal tissue location, where the catheter includes at leastfirst and second electrodes. First and second electrical signals may beprovided to the first and second electrodes, respectively. Accordingly,the relative response of the first and second electrodes may beidentified. Further, an output may be generated based on a change in therelative response. For instance, if both electrodes are initially in ablood pool (i.e., heart chamber) and one of the electrodes contacts awall of the chamber, the impedance of the contacting electrode maychange relative to the non-contacting electrode. In contrast, if bothelectrodes pass into a smaller surrounding area (e.g., a vein orartery), each electrode may experience a change in impedance such that arelative impedance may remain substantially unchanged.

The system and method may include providing first and second signalshaving equal frequency and phase, wherein an amplitude of one of thefirst and second signals is adjustable. Such individual adjustment ofthe amplitude of one of the signals may allow for matching the impedanceof the first and second electrodes. In this regard, it will be notedthat the resistivity of blood may change during the cardiac cycle. Thatis, the flow of the blood may change the resistivity and thus themeasured impedance by several percent or more. Accordingly, it may bedesirable to match the impedances of the electrodes in order to accountfor dynamic changes in the electrical properties of local media. Statedotherwise, by matching the impedance of the electrodes, the electrodecatheter may be calibrated for local conditions. Such calibration mayinclude initially positioning the first and second electrodes at alocation within a blood pool (e.g., away from a wall in a heartchamber). The impedances of the first and second sensors may then bematched. Accordingly, common mode noise and impedance may be cancelledbetween the first and second electrodes thereby accounting for localvariances. The catheter may be moved until one of the electrodescontacts the surface. An impedance measurement of the contact may thenbe obtained that is substantially free of local variance.

The foregoing and other aspects, features, details, utilities, andadvantages of the present invention will be apparent from reading thefollowing description and claims, and from reviewing the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of an exemplary catheter systemwhich may be implemented to access internal patient tissue for mappingand/or tissue ablation procedures.

FIG. 1 a is a detailed illustration of the patient's heart in FIG. 1,showing the electrode catheter after it has been moved into thepatient's heart.

FIG. 2 is an exemplary catheter that may be utilized with the system ofFIG. 1

FIGS. 3 a-3 c are exemplary perspective views of relative positions ofan electrode catheter and sheath.

FIG. 4 is functional block diagram showing the exemplary catheter systemof FIG. 1 in more detail.

FIG. 5 is a functional diagram of an impedance determination circuit.

FIG. 6 illustrates one embodiment of a protocol that may be used toassess local impedance of a catheter electrode.

FIG. 7 is an exemplary block diagram showing impedance measurement forcontact sensing and tissue sensing.

FIG. 7 a is a circuit equivalent of the block diagram of FIG. 7.

FIG. 8 a illustrates an electrode catheter disposed in blood pool out oftissue contact for calibration.

FIG. 8 b illustrates an electrode catheter contacting patient tissueafter calibration.

FIG. 9 illustrates a measurement circuit for single ended measurement offirst and second electrodes.

FIG. 10 illustrates one embodiment of a protocol that may be used tocalibrate an electrode catheter for local variances.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a diagrammatic illustration of an exemplary electrode cathetersystem 10 which may be implemented to assess and map internal patienttissue. Further, the system is operative to assess electrode-tissuecontact to assist in performance of a tissue ablation procedure for apatient 12. Catheter system 10 may include a guiding introducer having asheath 8, which may be inserted into the patient 12. The sheath 8 mayprovide a lumen for the introduction of a catheter 14 which may bedisposed beyond the distal insertion end of the sheath 8, e.g., forforming ablative lesions inside the patient's heart 16. During anexemplary ablation procedure, a user (e.g., the patient's physician or atechnician) may insert the sheath of a guiding introducer into one ofthe patient's blood vessels 18, e.g., through the leg (as shown inFIG. 1) or the patient's neck. The user, guided by a real-timefluoroscopy imaging device (not shown), moves the sheath 8 into thepatient's heart 16 (as shown in more detail in FIG. 1 a). When thesheath 8 of the guiding introducer reaches the patient's heart 16, theelectrode catheter 14 may be extended through a lumen of the sheath 8such that the electrode catheter 14 may be guided to a desired locationwithin the heart to perform, for example tissue mapping and/or tissueablation. In tissue mapping procedures, a model of the heart may begenerated on an output display 11, which may be utilized for subsequentcatheter guidance to perform, for example, an ablation procedure. One ormore additional catheters 14 a may also be utilized during mappingand/or subsequent procedures.

FIG. 2 illustrates one embodiment of an electrode catheter system 36with an electrode catheter 14 that may be selectively extended from thedistal end portion of the sheath 8 of the guiding introducer. As usedherein and commonly used in the art, the term “distal” is used generallyto refer to components of the catheter system, such as the tip electrode20, located toward the insertion end of the of the ablation catheter 14(i.e., toward the heart or other target tissue when the catheter is inuse). In contrast, the term “proximal” is used generally to refer tocomponents or portions of the catheter that are located or generallyorientated toward the non-insertion end of the catheter (i.e., away fromor opposite the heart or other target tissue when the catheter is inuse).

The sheath 8 is a tubular structure defining at least one lumen orlongitudinal channel. The sheath 8 is used in conjunction with thecatheter 14 to introduce and guide the catheter 14 to the targetinternal tissue area. The catheter 14, however, may be used alone orwith other guiding and introducing type devices depending on theparticular procedure being performed. As shown in FIG. 2, the catheterincludes a tubular body or shaft 6 extending from the connector, throughthe sheath 8, and out of the lumen at the distal end of the sheath 8. Inone implementation, the sheath 8 and shaft 6 are fabricated with aflexible resilient material. The sheath and the components of thecatheter are preferably fabricated of materials suitable for use inhumans, such as polymers. Suitable polymers include those well known inthe art, such as polyurethanes, polyether-block amides, polyolefins,nylons, polytetrafluoroethylene, polyvinylidene fluoride, andfluorinated ethylene propylene polymers, and other materials. In theparticular ablation system configuration of FIG. 2, the sheath 8 isconfigured to receive and guide the ablation catheter within an internallumen to the appropriate location in the heart once the sheath ispre-positioned in the appropriate location.

The electrode catheter 14 of the exemplary embodiment includes a tipelectrode 20 and a plurality of ring electrodes 22 a-n (referred tocollectively as electrodes 22). Though shown as utilizing a plurality ofring electrodes, it will be noted that other electrodes may be utilizedas well. For instance, spot electrodes or segmented ring electrodes maybe utilized. These electrodes 20, 22 may be implemented to electricallymap the myocardium 24 (i.e., muscular tissue in the heart wall). In thisregard, information from the electrodes may be utilized to a createrealistic model of cardiac chamber geometries or models of otherinternal tissue depending on the particular procedure being performed.As noted, such a model may be displayed on a user output 11 (See FIG. 1)for use in guiding the catheter 14, for example, during an ablationprocedure performed after mapping.

To create the model, a 3D location system such as the NavX™ navigationand visualization system of St. Jude Medical may be used. In such asystem, two or more external patient electrode patches 46 (only oneshown) are applied on one or more locations on the body. An electricalsignal is transmitted between the patches, and one or more electrodes ofone or more catheter within the heart sense the signal. The system 10collects electrical data from the catheters and uses this information totrack or navigate their movement and construct three-dimensional (3-D)models of the chamber. Additionally a physician may sweep thecatheter(s) 14 across the heart chamber during data collection tooutline the structures and relay the signals to the computer system,which generates the 3-D model. The resulting model may then be utilizedto, for example, guide the catheter 14 to one or more locations in theheart where treatment is needed.

Such a system allows for the creation of detailed internal models at thetime of study and/or performance of an internal procedure. This is, thesystem is operative to generate substantially real-time models. Such asystem avoids the possible challenges of imaging technologies that relyon models created prior to a time when an internal procedure isperformed. As may be appreciated, such previously created models may notreflect subsequent changes to the modeled tissue such as changes inposture and/or fluid loading.

While providing detailed models of internal tissue structure such as theheart, there are potential drawbacks of modeling systems that transmitelectrical signals from one or more external patches to an electrode(s)disposed within tissue of interest. For instance, if a catheterelectrode (e.g., tip electrode 20) that is utilized to receive thetransmitted electrical signals remains within the sheath 8 during thesampling process, gathered data may be distorted. However, suchdistorted data may yield a plausible, yet erroneous, model and/orerroneous location of the electrode and catheter on the display of theresulting model. Accordingly, an indicator of the position of theelectrode(s) of the catheter relative to the sheath is useful. Forinstance, an indication of an electrode being at the threshold exitlocation relative to the sheath may be useful.

It may also be useful to know how far a catheter is extended beyond asheath in order to determine how to guide the catheter. As will beappreciated, the catheter may be guided by a guide wire(s) thatbend/deflect an end portion of the catheter. Based on the compliance ofthe catheter shaft and the length of the portion extending beyond thesheath, a fulcrum may be determined about which the catheter will bendor deflect. Accordingly, knowledge of the length of the catheter may behelpful for determining how to best approach a tissue area of interest.Further, knowledge of when an electrode (e.g., tip electrode 20 and/orone or more ring electrodes 22) has just exited from the sheath allowsthe sheath exit site to be registered within a navigation model.

As may be appreciated, markings may be provided on proximal portions ofthe sheath 8 and/or the shaft 6 of the catheter 14 that may provide anindication of the relative positions of the distal tips of thesemembers. However, due to the bending and or compression of the sheath 8and/or catheter shaft 6 from routing these members to a tissue area ofinterest, such markings may not provide an accurate indication of therelative positions of the distal ends of these members. Accordingly, anindependent indicator of the distal end of the catheter 14 relative tothe distal end of the sheath 8 is desirable.

Such a indication may be provided by dedicated sensors interconnected tothe distal ends of the sheath 8 and catheter 14. However, due to thespace limitations of guided introducers and catheters, such dedicatedsensors may not provide an optimal solution. Presented herein, is asystem and method that allows for utilizing one or more existingelectrodes of the catheter 14 to provide an indication of the relativeposition of the distal end of the catheter 14 to the sheath 8. Such anindication may be provided for any catheter that passes through thesheath and includes at least one electrode.

Generally, it has been recognized that the environment surrounding acatheter electrode alters the impedance of the electrode in response toan applied electrical signal. For instance, the impedance may be higherwhen the electrode is within a confined/small volume structure (e.g., asheath or vein) as opposed to when the electrode is in a structurehaving a larger volume. That is, in the small volume structure, there isless surrounding fluid (e.g. blood) than in a larger structure such as aheart chamber. Stated otherwise, the local resistive capacity and/orcapacitive capacity of the surrounding blood varies with volume, whichalters the local impedance of the electrode fluid interface.

Accordingly, it as been recognized that the local impedance of acatheter electrode changes significantly based on its relative positionto the sheath 8. For instance, FIGS. 3A, 3B and 3C illustrate threerelative positions of a distal end of a catheter 14 to a sheath 8. Asshown in FIG. 3A, the catheter 14 is fully encased with the sheath 8. InFIG. 3B a tip surface of the catheter 14 is extending beyond the distalend of the sheath 8. As shown, this tip surface of the catheter 14 isformed by the tip electrode 20 that is connected to the catheter 14. InFIG. 3C, the catheter 14 is extended further beyond the sheath 8 suchthat the tip electrode 20 is disposed entirely beyond the sheath. Ofnote, FIG. 3C also illustrates the plurality of ring electrodes 22disposed along a portion of the length of the catheter 14.

As noted, the local impedance of the tip electrode 20 (or any otherelectrode) varies significantly based on its surroundings. For instance,when the tip electrode 20 is fully encased in the sheath, as shown inFIG. 3A, the tip electrode 20 may have, for example, a local impedanceof 300 Ohms. When partially exposed, as shown in FIG. 3B, the tipelectrode 20 may have a local impedance of 100 Ohms. Finally, whenentirely exposed from the sheath 8, as shown in FIG. 3C, the tipelectrode 20 may have a local impedance of 70 Ohms. In this regard,there is an impedance difference of over 4:1 between the condition wherethe tip electrode 20 is encased within the sheath 8 and where the tipelectrode 20 has completely exited the sheath 8. Accordingly, bymonitoring the impedance of the tip electrode 20, an indication of thelocation of the distal end of the catheter 14 relative to the distal endof the sheath 8 may be provided. Further, such an indication may beprovided utilizing existing componentry (e.g., electrodes) of thecatheter 14.

FIG. 4 is a high-level functional block diagram showing the cathetersystem 10 in more detail as it may be implemented to assess position ofthe electrode catheter 14 relative to the distal end of the sheath 8. Itis noted that some of the components typical of conventional tissueablation systems are shown in simplified form and/or not shown at all inFIGS. 1 and 4 for purposes of brevity. Such components may neverthelessalso be provided as part of, or for use with the catheter system 10. Forexample, electrode catheter 14 may include a handle portion, afluoroscopy imaging device, and/or various other controls, to name onlya few examples. Such components are well understood in the medicaldevices arts and therefore further discussion herein is not necessaryfor a complete understanding of the invention.

The exemplary catheter system 10 may include a generator 40, such as,e.g., AC current generator and/or a radio frequency (RF) generator,which in the present embodiment provides an electrical signal(s) to theelectrode(s) of the catheter 14 (as illustrated by wires 44) forelectrode position measurement, electrode contact measurement and/orablation purposes. A measurement circuit 42 is electrically connected tothe tip electrode 20. The electrode catheter 14 may also be electricallygrounded, e.g., through grounding patch 46 affixed to the patient's armor chest (as shown in FIG. 1).

Generator 40 may be operated to emit electrical energy to the tipelectrode 20 of catheter 14. Generally, frequencies from 1 KHz to 500KHz are suitable for this measurement. The measurement circuitry may bepart of an ablation generator system, however, the impedancemeasurement(s) may be made with low-level signals such as, for example,10 micro-amps. The resulting impedance at the electrode in response tothe applied signal may be measured or monitored on a continuous basisusing the measurement circuit 42. In one embodiment, the measurementcircuit 42 may be a conventionally availableresistance-capacitance-inductance (RCL). Still other measurementcircuits 42 may be implemented and the invention is not limited to usewith any particular type or configuration of measurement circuit. In anycase, the impedance measurements may be used to determine an indicationof the position of the tip electrode 20 (or other electrode) in relationto the sheath 8. This position may then be conveyed to the user inreal-time to indicate, for example, if an electrode is exposed such thata mapping procedure may continue.

In an exemplary embodiment, the measurement circuit 42 may beoperatively associated with a processor 50 and memory 52 to analyze themeasured impedance. By way of example, processor 50 may determine aninitial impedance at a catheter position that is known to be within thesheath 8. The processor may then sample subsequent impedancemeasurements to determine a change of the measured impedance. In anexemplary embodiment, impedance changes based on varying positions of,for example differently sized catheters, may be predetermined, e.g.,during testing for any of a wide range of catheters and sheaths. Theimpedance changes may be stored in memory 52, e.g., as tables or othersuitable data structures. The processor 50 may then access the tables orequations in memory 52 and determine a change in impedance (e.g., froman initial impedance) that indicates that the electrode is at leastpartially or fully exposed outside of the sheath. An indication of therelative position may be output for the user, e.g., at display device54. As will be appreciated, the process may also be reversed todetermine when a catheter has been withdrawn into a sheath.

In a further exemplary embodiment, the generator 40 may be operated toemit electrical energy, e.g., electrical signals, to the tip electrode20 and at least one of the ring electrodes 22. For instance, thegenerator 40 may emit separate drive signals to the tip electrode 20 andthe first ring electrode 22 a. See for example FIG. 3C. The resultingimpedance at each electrode 20, 22 a in response to the applied signalsmay be measured using the measurement circuit 42. In such an embodiment,initial impedance values of the two electrodes 20, 22 a, e.g., whenfully disposed within the sheath 8, may be identified. Accordingly, theprocessor may generate a relative value of the impedances of the firstand second electrodes 20, 22 a. If an impedance of one of the electrodessubsequently changes, for example in conjunction with movement of thecatheter, the impedance of the other electrode may remain substantiallythe same. In this instance, the relative value of the impedances maychange. Accordingly, if the relative output changes by a sufficientdegree, the processor may generate an output for the display.

FIG. 5 illustrates another exemplary embodiment of a system formonitoring local impedance of one or more electrodes of a catheter. Inthis exemplary embodiment, impedance is not measured directly, rather,it is demodulated from the electrical signal provided to the electrode.As shown, one method is to pass a low level AC current, for example, 10micro-amps at 40 kHz, through the electrode to be analyzed. In thisregard, the generator 40 may provide the desired electrical signal I₁ tothe tip electrode 20. As will be discussed herein, the generator 40 mayalso provide additional electrical signals to additional electrodes. Thereturn path for electrical signal I₁ applied to the tip electrode isconveniently a body surface electrode, such that no other intra-cardiacelectrodes are required for implementation. A differential amplifier 48is provided for use in determining an indication of the impedance of theelectrode being analyzed. Accordingly, an amplifier input is alsoconnected to the analyzed electrode via a tap on the wire 44 a carryingthe electrical signal I₁ to the tip electrode 20. The amplifier isreferenced to the body surface electrode 46 or to another body surfaceelectrode. The resulting amplitude measured on the tip electrode 20, orother electrode as the case may be, may be recovered by synchronousdemodulation of the driven current frequency from circuitry (e.g.,processor 50) subsequent to the amplifier 48. The results of thedemodulated signal will primarily reflect the very local ambientimpedance of the electrode. The far field impedance of the currentreturning to the body surface will be very small because it is greatlyspread out. The net result is that the demodulated signal/measured valueis highly weighted toward the impedance of the electrode-fluid interfaceand bulk impedance immediately surrounding the electrode 20.Accordingly, the measured value may be monitored to identify changesthat indicate the tip electrode 20 is entering or exiting the sheath.

FIG. 6 illustrates one embodiment of a sheath exit sensing protocol 100to determine when a catheter including at least one electrode exits asheath of a guiding introducer. Initially, a sheath of a guidingintroducer is guided (102) to an internal tissue location of interest.For instance, the sheath may be guided to a chamber of a patient'sheart. Once the sheath is properly positioned, an electrical signal maybe applied (104) to an electrode of a catheter disposed within aninternal lumen of the sheath. Further, an initial impedance of theelectrode in response to the electrical signal may be measured (106).Such measurement may be a direct measurement as discussed in relation toFIG. 4 or an indirect measurement as discussed in relation to FIG. 5.The catheter may be displaced (108) relative to the sheath while theimpedance of the electrode is monitored (110). If a predetermined changein the impedance is detected, an output (112) indicating a change in therelative position of the electrode to the sheath may be provided. Forinstance, and output indicating the electrode has partially passed orfully passed out of the introducer may be provided to a display.

In addition to utilizing local impedances to identify changes insurrounding structure, such local impedances may also be utilized tocalibrate a catheter for contact assessment. As will be appreciated, itgenerally known that contact impedance of an electrode may be utilizedto determine if endocardial contact is achieved or how vigorous thecontact is relative to, for example a catheter born electrode freefloating in blood (e.g., in a heart chamber). Assessing a contact orcoupling condition between the electrode catheter 14 and target tissue24 based on impedance measurements at the electrode-tissue interface maybe better understood with reference to FIGS. 7 and 7 a. FIG. 7 is amodel of the electrode catheter 14 in contact with (or coupled to)target tissue (e.g., specific myocardium tissue 24). The electrodecatheter 14 is electrically connected to the generator 40 (e.g., an RFgenerator). In an exemplary embodiment, the circuit may be completedthrough the target myocardium tissue 24, showing that current flowsthrough the blood, myocardium, and other organs to the referenceelectrode, such as a grounding patch 46 on the patient's body. See FIG.1.

As described above, the generator 40 may be operated to generateelectrical energy for emission by the electrode catheter 14. Emissionsare illustrated in FIG. 7 by arrows 60. To avoid a risk of inducing anarrhythmia during contact or coupling assessment, it is desirable to usea low amount of current and power. A presently preferred range forfrequencies between 1 KHz and 500 KHz and a current less than 10micro-amps.

The frequency choice is mostly based on physiological aspect andengineering aspect and is within the purview of one of ordinary skill inthe art. For physiological aspect, lower frequencies can introducemeasurement errors due to electrode-electrolyte interface. Whenfrequency goes higher to MHz range or above, the parasitic capacitancecan become significant. It is noted, however, that the invention is notlimited to use at any particular frequency or range of frequencies. Thefrequency may depend at least to some extent on operationalconsiderations, such as, e.g., the application, the type of targettissue, and the type of electrical energy being used, to name only a fewexamples.

Assuming, that a desired frequency has been selected for the particularapplication, the model shown in FIG. 7 may be further expressed as asimplified electrical circuit 62, as shown in FIG. 7 a. In the circuit62, generator 40 is represented as an AC source 64. The capacitance andresistance at the blood-tissue interface dominate impedance measurementsat low frequency operation such as may be used for assessingelectrode-tissue contact. Accordingly, other capacitive, inductive, andresistive effects may be ignored and the capacitive-resistive effects atthe blood-tissue interface may be represented in circuit 62 by aresistor-capacitor (R-C) circuit 66.

The R-C circuit 66 may include a resistor 68 representing the resistiveeffects of blood on impedance, in parallel with a resistor 70 andcapacitor 72 representing the resistive and capacitive effects of thetarget tissue 24 on impedance. When the electrode catheter 14 has no orlittle contact with the target tissue 24, which may include interstitialfluid spaces 23 and/cell membranes 25, resistive effects of the bloodaffect the R-C circuit 66, and hence also affect the impedancemeasurements. As the electrode catheter 14 is moved into contact withthe target tissue 24, however, the resistive and capacitive effects ofthe target tissue 24 affect the R-C circuit 66, and hence also affectthe impedance measurements.

The effects of resistance and capacitance on impedance measurements maybe better understood with reference to a definition of impedance.Impedance (Z) may be expressed as:

Z=R+jX

where:

-   -   R is resistance from the blood and/or tissue;    -   j an imaginary number indicating the term has a phase angle of        +90 degrees; and    -   X is reactance from both capacitance and inductance.

It is observed from the above equation that the magnitude of thereactance component responds to both resistive and capacitive effects ofthe circuit 62. This variation corresponds directly to the level ofcontact or coupling at the electrode-tissue interface, and therefore maybe used to assess the electrode-tissue contact or coupling. By way ofexample, when the electrode catheter 14 is operated at a frequency of100 kHz and is primarily in contact with the blood, the impedance ispurely resistive and the reactance (X) is close to 0 Ohms. When theelectrode catheter 14 contacts the target tissue, the reactancecomponent becomes negative. As the level of contact or coupling isincreased, the reactance component becomes more negative.

Measurement circuitry may be designed to measure either or bothcomponents (R and/or X) of the above-noted equation, or equivalentlytheir complex arithmetic equivalents: the magnitude and phase angle.When in the blood pool, the measurement almost entirely comprisesresistance, whereas, particularly at higher contemplated frequencies,there is a small but discernable capacitive component when an electrodecontacts a tissue boundary. The techniques taught herein may be appliedusing the resistive, reactive, magnitude, or phase angle of impedance.

Considering just the resistive component and ignoring the electrodeimpedance effects per se for a spherical electrode, (e.g., the distalend of tip electrode 20), the contact impedance is resistive and can beapproximates as:

ρ/(4πr)

where r is the radius of the electrode and ρ is the medium resistivityof blood in this case.

If 20% of the electrode is in contact with tissue having a resistivityof three times that of blood, the impedance of the electrode 20 willincrease by about 15%. This can be calculated by treating the apparentresistance of the two surfaces as parallel resistances. Such a change ofimpedance may be detected and an output indicative of such contact maybe generated. However, in an instance where the tissue is only twice asresistive as the blood, a 20% contact of the electrode with the tissuewill only result in about an 11% increase of the impedance of theelectrode 20. In such instances of smaller increases due to contact, theimpedance increases may fall into an expected variance range ofimpedance values.

As will be appreciated, the local impedance of the surrounding bloodvaries based on one or more physiological factors. One such factor isthe cardiac stroke signal, which may cause changes of 5%-10% of the baseimpedance. That is, impedance may change throughout the cardiac cycle.Resistivity also changes some with ventilating and or flow rate ofblood. Thus, resistivity of blood in the above equation cannot be reliedon to be a constant. Further, failure to account for such physiologicalfactors or ‘local variance’ may result in false positive and/or falsenegative indications of electrode-tissue contact.

Utilizing the system outlined above in relation to FIG. 5 a process isprovided that that independently samples the local resistivity inessence and dynamically cancels changes in it. The method furthercancels common mode noise and impedance thereby affording a contactsensor that produces fewer false positive/negative contact indications.In this regard, the system may be calibrated to account for variances inlocal impedance. The method is outlined in relation to FIGS. 5, 8 a and8 b. As illustrated in FIG. 5 two current sources I₁ and I₂ are providedwhich provide electrical signals to the tip electrode 20 and a ringelectrode 22. These current sources I₁ and I₂ are of identical frequencyand phase and at least one of the current sources I₁ and I₂ has aprogrammable or trimmable amplitude. One source (I₁) is connected to theelectro-cardio electrode, whose contact status is desired, for exampletip electrode 20, and the second source (I₂) is connected to a referenceelectrode, for example ring electrode 22 a, which is ideally within 2-20mm of the tip electrode 20. The current for the two sources may bereturned to a common patch on the body surface as shown, or it may bereturned to a blood pool electrode. Since sensing is not done on thisreturn electrode, local resistivity changes in its proximity will haveno impact. Further, the differential amplifier 48 can be set to highgain in this method, maximizing common mode rejection and minimizingnoise.

In-vitro or in-vivo at the start of a study, a calibration is done withthe catheter 14 away from a wall or boundary. See FIG. 8 a. Thecalibration consists of trimming or programming one of the currentsources I₁ and I₂ until commonly zero volts are demodulated from theimpedance circuit. Thus, if the tip electrode 20 has a larger surfacearea than the ring electrode 22 a, I₁ may be trimmed to deliver morecurrent such that the potential created equals that from I₂ driving thering electrode 22 a. With this calibration, local resistivity changescommon to both electrodes are now be cancelled out. Now only whenelectrode 20 or 22 a incurs a change, such as contact, and the otherdoes not, will a potential register out of the impedance circuit.Accordingly, when such a potential is identified, this contact may beregistered with, for example, a visualization and/or navigation system.Of note, it may be beneficial to implement the system with a segmentedring electrode that is electrically divided into, for example, three orfour segments. If separate differential measurements are made betweenthe tip electrode and each segment, then in the case of a catheterlaying along tissue, at least one of the segments of the segmented ringelectrode may not be contacting tissue. That is, one or more segmentsmay be facing the fluid/blood pool. IF the tip electrode is contactingthe tissue, a high differential will register between the tip electrodeand the blood facing segment(s). This may provide a robust indication ofcontact. A similar system may be implemented using spot electrodes.

Such ‘local normalization’ may be performed when the catheter isdisposed near a contact surface (e.g., within a fluid/blood pool), whichmay be determined using a substantially real-time model as discussedabove. Generally, the differential of the electrode outputs will besmall but may be non-zero at this location. However, before an operator(or robotic system) makes a move to contact the surface, this ‘offset’may be measured and subsequently subtracted as the catheter approachesthe surface for ablation or other purposes. This removal of the offsetvalue reduces the likelihood of false positives or negatives occurringwhen utilizing the tip electrode (or other electrode) as an endocardialsurface contact sensor.

In another exemplary embodiment, a further system and method foraccounting for local variance is provided. As illustrated in FIG. 9,single ended measurements are made for separate electrodes of thecatheter. For instance, a first measurement T is made for the tipelectrode 20 and a second measurement R is made for a ring electrode.The measurements may be made individually utilizing different amplifiercircuitry as shown. In such an arrangement, subsequent processing may beperformed digitally (e.g., in software). In any case, the measurements Tand R may be utilized to calibrate the system for subsequent contactsensing. In this regard, a differential may be determined for theelectrodes in a blood pool. For instance:

D _(b) =T _(b) −R _(b)≈0

In this regard, one of the measurements may be weighted (e.g., scaled)relative to the other measurement such the differential is substantiallyzero. At the same time, a common mode or nominal average may becomputed:

A _(b)=(T _(b) +R _(b))/2

Going forward, the average of the blood pool A_(b) may be utilized tonormalize subsequent measurements. For instance:

(A _(b) /A)^(x) ·D

The value x may be 1.0 or any other value that optimizes sensitivity. Dis the differential value. In this instance, if the average value of Aincreases substantially (e.g., due to tissue contact of one of theelectrodes) relative to the calibration value (i.e., A_(b)) the resultsmay be derated. The system further minimizes sensitivity to falsepositives. With this system, the higher the average value, the lessweight is applied to the difference. While such derating may be linearwith the exponent x=1 in the above equation, non-linear equations may beutilized as well.

FIG. 10 illustrates one embodiment of a calibration protocol (200) forcalibrating an electrode catheter for local variance. Initially, acatheter having at least first and second electrodes is guided (202) toa fluid pool that is out of contact with a boundary of the fluid pool.See FIG. 8 a. Such guiding may be assisted by various imaging devices.Individual electrical signals may be applied (204) to the electrodes andan initial impedance of the first and second electrodes may be obtained(206). This may entail directly measuring the impedance of eachelectrode, indirectly measuring the impedance of each electrode and/oridentifying a relative/differential impedance of the electrodes. One ofthe electrical signals may be adjusted (208) to substantially match orequalize the impedance of the two electrodes. Once the impedances arematched, the catheter is calibrated and may be moved into contact (210)with patient tissue. See FIG. 8 b.

Although three embodiments of a process for detecting local impedanceand two applications of the measured local impedance have been describedabove with a certain degree of particularity, those skilled in the artcould make numerous alterations to the disclosed embodiments withoutdeparting from the spirit or scope of this invention. For example, itwill be appreciated that the other circuits may be designed fordetecting/measuring a local impedance of a catheter electrode. However,an important feature of this invention is the recognition that thislocal impedance changes based on the surroundings of the electrode. Inthis regard, such changes may be utilized for other applications. Forinstance, indications of a change of impedance may be utilized to guidea catheter into small volume structure (e.g., veins or arteries) fromlarger volume structure (e.g., heart chambers) or vice versa. Further,it will be noted that all directional references (e.g., upper, lower,upward, downward, left, right, leftward, rightward, top, bottom, above,below, vertical, horizontal, clockwise, and counterclockwise) are onlyused for identification purposes to aid the reader's understanding ofthe present invention, and do not create limitations, particularly as tothe position, orientation, or use of the invention. Joinder references(e.g., attached, coupled, connected, and the like) are to be construedbroadly and may include intermediate members between a connection ofelements and relative movement between elements. As such, joinderreferences do not necessarily infer that two elements are directlyconnected and in fixed relation to each other. It is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative only and not limiting.Changes in detail or structure may be made without departing from thespirit of the invention as defined in the appended claims.

1. A method for monitoring the position of an electrode relative to anintroducer, comprising: providing an electrical signal to an electrodeattached to a body of a catheter at least partially disposed within aninterior of an introducer; measuring an initial impedance response ofthe electrode to the electrical signal when the electrode is disposedwithin the interior of the introducer and a subsequent impedanceresponse of the electrode to the electrical signal upon displacement ofthe catheter relative to the introducer, wherein said measuring isperformed using an impedance measurement module; and processing the saidinitial impedance response and said subsequent impedance response to:identify an impedance change between said initial impedance response andsaid subsequent impedance response; compare said impedance change topredetermined impedance threshold information stored in memory; and uponsaid change exceeding at least one predetermined impedance thresholdlevel, generating an output indicative of a displacement of saidelectrode relative to an end of said introducer.
 2. The method of claim1, further comprising: prior to comparing said change to said at leastone predetermined impedance threshold level, scaling said change basedon a level of said initial impedance response.
 3. The method of claim 1,further comprising: operating an electrical power source to provide afirst electrical signal to a first electrode attached to the body of thecatheter and provide a second electrical signal to a second electrodeattached to the body of the catheter, wherein measuring an initialimpedance response comprises measuring a first impedance response of thefirst electrode and measuring a second impedance response of the secondelectrode, and wherein said initial impedance response is a firstrelative response of the first and second impedance responses.
 4. Themethod of claim 3, further comprising: adjusting the amplitude of atleast one of said first and second electrical signals to match saidfirst impedance response and said second impedance response.
 5. Themethod of claim 3, wherein said subsequent response comprises a secondrelative response of the first and second electrodes to said first andsecond signals, respectively, upon said displacement.
 6. The method ofclaim 5, wherein identifying said change comprises identifying a changebetween said first relative response and said second relative response.7. The method of claim 1, wherein generating said output comprisesgenerating an output indicating the electrode is at least partiallyexposed outside of the introducer.
 8. The method of claim 1, whereincomparing said impedance change to at least one predetermined thresholdlevel further comprises: accessing predetermined threshold informationthat corresponds to a physical configuration of at least one of thecatheter and the introducer.
 9. The method of claim 1, wherein measuringsaid initial and subsequent impedance responses comprises: measuring theelectrical signal applied to said electrode; and demodulating saidimpedances from said the electrical signal as applied to said electrode.10. A medical system comprising: an electrical power system configuredto provide a first electrical signal to a first electrode of a catheterat least partially disposed within an interior of the introducer; animpedance measurement module configured to measure a first impedance ofthe first electrode in response to the first electrical signal while thefirst electrode is located at a first position within the introducer andmeasure a second impedance of the first electrode upon displacement ofthe catheter relative to the introducer; a data structure configured tostore predetermined impedance threshold information associated with thecatheter and the introducer; and a processor configured to: identify animpedance change between said first impedance and said second impedance;compare said impedance change to predetermined threshold informationstored in said data structure; and upon said change exceeding at leastone predetermined threshold level, generate an output indicative of arelative displacement of said electrode to an end of said introducer,wherein the electrode is located at a second position relative to theintroducer.
 11. The system of claim 10, wherein said processor isfurther configured to: prior to comparing said change to saidpredetermined impedance threshold information, scale said change basedon a level of said first impedance.
 12. The system of claim 10, whereinthe impedance measurement module directly measures the impedance of thefirst electrode.
 13. The system of claim 10, wherein the impedancemeasurement module indirectly measures the impedance of the firstelectrode.
 14. The system of claim 13, wherein said processor identifiesa differential signal between said first electrical signal as appliedand a second electrical signal as measured on a second electrode, wheresaid second electrical signal is a return signal of said firstelectrical signal.
 15. The system of claim 13, wherein the impedancemeasurement module includes: a differential amplifier having first andsecond inputs connected to the first electrode and a second electrode,respectively.
 16. The system of claim 15, further comprising: ademodulator for demodulating an output of the differential amplifier.17. A method for monitoring the position of an electrode relative to anintroducer, comprising: providing first and second electrical signals tofirst and second electrodes, respectively, attached to the body of thecatheter, wherein the catheter is at least partially disposed within aninterior of an introducer; measuring first and second impedanceresponses of the first and second electrodes to the first and secondelectrical signals, respectively, to generate an initial relativeresponse while the catheter is located at a first position within thecatheter; identifying a change in the initial relative response, upondisplacement of the catheter from said first position to a secondposition; scaling said change based on a level of the initial responseto generate a scaled change of impedance; and generating an outputindicative of a displacement of at least one of said electrodes relativeto an end of said introducer based on said scaled change of impedance.18. The method of claim 17, further comprising: adjusting the amplitudeof at least one of the first and second electrical signals to match thefirst impedance response and the second impedance response.
 19. Themethod of claim 17, wherein generating said output further comprises:comparing said scaled impedance change to predetermined impedancethreshold information stored in memory; and upon said scaled impedancechange exceeding at least one predetermined impedance threshold,generating said output.
 20. The method of claim 19, wherein comparingfurther comprises: accessing predetermined threshold information thatcorresponds to a physical configuration of at least one of the catheterand the introducer.
 21. The method of claim 19, wherein generating saidoutput comprises generating an output indicating the at least oneelectrode is at least partially exposed outside of the end of theintroducer.